When manufacturing a disk drive, servo sectors 20–27 are written to a disk 4 which define a plurality of radially-spaced, concentric data tracks 6 as shown in the prior art disk format of FIG. 1. Each servo sector (e.g., servo sector 24) comprises a preamble 108 for synchronizing gain control and timing recovery, a sync mark 110 for synchronizing to a data field comprising coarse head positioning information such as a track number (TKID 111), a servo wedge ID track including a servo wedge number to provide circumferential position data, and servo bursts 114 which provide fine head positioning information. During normal operation the servo bursts 114 are processed by the disk drive in order to maintain a head over a centerline of a target track while writing or reading data.
External servo writers are typically used to write the servo sectors 2 to the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer or optical encoder, to ensure the servo sectors are written at the proper radial location from the outer diameter of the disk to the inner diameter of the disk. However, external servo writers are expensive to operate, require a clean room environment, and are a very significant cost in the disk drive manufacturing process.
The prior art has suggested various self servo-writing methods wherein the internal electronics of the disk drive are used to write the servo sectors independent of an external servo writer to reduce the reliance on external servo writers. For example, U.S. Pat. No. 5,668,679 teaches a disk drive which performs a self-servo writing operation by writing a plurality of spiral reference patterns to the disk which are then processed to write the servo sectors along a circular path. The spiral patterns are written “open loop” by seeking the head from an outer diameter of the disk to an inner diameter of the disk. The disk drive calibrates acceleration/deceleration impulses to seek the head from the outer to inner diameter in a desired amount of time. Accurate radial positioning of the spiral patterns assumes the calibration process is accurate and that the calibrated acceleration/deceleration impulses will generate a repeatable response over multiple seeks.
However, the calibration process will inevitably exhibit some degree of error and the dynamics of the disk drive will change between seeks inducing errors in the radial position of the spiral reference patterns. Dynamic errors which degrade the spiral patterns written during an open loop seek include vibration of the HDA, flutter and non-repeatable run-out of the disk and spindle bearings, stiction and non-repeatable run-out of the pivot bearings, windage on the head and arm, and flex circuit bias, windage, vibration, and temperature. Errors in writing the spiral patterns will propagate to the servo sectors, thereby degrading the operating performance of the disk drive and reducing the manufacturing yield.
Other attempts have been made to utilize an external servo writer in writing spiral reference patterns to a disk and then utilizing the control circuitry of the disk drive itself to write the servo sectors based upon the previously written spiral patterns. In this combination, spiral patterns may be written with a higher degree of accuracy utilizing the external servo writer and thereafter servo sectors may be written utilizing the disk drive itself to minimize the cost and expense related to the use of an external servo writer.
Nonetheless, even with the use of an external servo writer to aid in writing spiral reference patterns to the disk, previously employed methods unfortunately require a relatively large amount of in order to write the spiral patterns and, further, due to the thermal expansion and/or contraction of the head disk assembly (HDA) along with other error sources there is still a relatively large amount of clock track drift introduced into the process, which results in errors in the written spiral reference patterns and consequently the written servo sectors to the disk.
For example, one previously employed method utilizes an external servo writer to write a single frequency reference track at the inner diameter (ID) of a disk and a single frequency reference track at the outer diameter (OD) of the disk. These reference tracks consist of a single frequency tone, one head width wide, bounded on both sides by bands of erased tracks to either side.
Under the control of the external servo writer, spiral reference patterns may be written. This method involves the external servo writer moving the actuator of the HDA toward the outer diameter, within the OD erase bands, to search for the OD reference track. While searching for the OD reference track, a track average amplitude (TAA) is read as the head of the actuator is incrementally stepped toward the OD reference track until the reference track signal is detected. Usually a threshold is counted to discriminate between the amplitude of an erase signal or the amplitude of a written reference track signal. The laser position of the external servo writer is then recorded. The actuator under the control of the external servo writer continues stepping until the TAA amplitude drops back below the threshold previously described and the laser position of the external servo writer is again recorded. The laser position is then averaged and this is regarded as the OD reference track center. The same process is employed for finding the reference track of the ID.
A spiral reference pattern may then be written between the OD reference track and the ID reference track. This process of finding the OD reference track and the ID reference track is employed every time a spiral reference pattern is to be written.
Unfortunately, this process takes a relatively large amount of time to find the OD and ID reference tracks in order to write the spiral pattern and still does not account for many of the variables associated with clock track drift, such as thermal expansion and/or contraction of the HDA, resulting in less accurate spiral patterns and consequently less accurate servo sectors subsequently written to the disk based on the spiral pattern.